This invention relates to microwave television systems including an antenna with appropriate electronic circuitry, and more particularly to a probe excited waveguide used as the antenna with the electronics contained directly within the waveguide.
Antennas in conjunction with appropriate electronic circuitry are utilized for both transmission and receiving of television signals. Most recently the MDS (Multi-point Distribution Systems) type of communications has been experiencing continuous growth. Such systems have been associated with pay television for transmission of movies, special programs, as well as for teaching purposes. The benefit of the MDS system is that a single receiver could be utilized for a number of locations with the output being distributed to each location. This is convenient for apartment houses, as well as homes in a development at a remote location from the source. The typical MDS system includes the transmitter, the program source, and the receiving system.
At the MDS transmitter there is provided an up-converter-amplifier driven by an exciter whose output is transmitted by an antenna. The exciter section can accept video and audio from any number of sources such as an earth sattelite receiver or studio equipment. The output of the modulator is a standard television signal. The frequency alloted to MDS transmission is approximately 2 GHz.
At the receiver, there is again employed an antenna for receiving the signal, a down-converter to lower the frequency of the MDS signal to a frequency that can be easily amplified and distributed to the MDS subscribers and a television receiver for displaying the received signal. Additional circuitry is also frequently included at the receiver end including a pre-selector, a pre-amplifier, as well as an IF amplifier.
Generally, the type of antennas that are recommended for use with an MDS system are the dish antenna, the corner reflector or the yagi. The requirement for a suitable antenna are that it achieve relatively high gain, and high directivity over a large frequency band. At the same time, the cost of the antenna is a critical factor, especially where the cost of the total installation is of importance as for private home installations.
One type of antenna which has achieved relatively high gain, high directivity over a large frequency band, and has nevertheless been available at low cost, is the disc-on-rod type antenna, excited by a launcher or elementary antenna. Such disc-on-rod type antenna has been described in my prior U.S. Pat. No. 2,955,287, issued on Oct. 4, 1960, and in my U.S. Pat. No. 3,015,821 issued on Jan. 2, 1962. In those patents, both of which are herein incorporated by reference, there is described an end fire radiator having a principal axis adapted to be energized by a launcher at its non radiating end for the transmission of energy of a desired wavelength in the direction of the axis. The electrically active components of the radiator consist of a plurality of substantially identical electrically conductive plates having a major dimension greater than .lambda./4 and less than .lambda./2 and spaced from each other a distance between .lambda./8 and .lambda./2, wherein .lambda. is the wavelength of the energy. The plane of the plates are normal to the axis to thereby form an elongated radiator.
A further modification of the basic disc-on-rod type of antenna is described in my prior U.S. Pat. No. 3,440,658 issued on Apr. 22, 1969 and also incorporated herein by reference. In that patent, there is described a combination type antenna including the disc-on-rod antenna in conjunction with a second antenna, the combination of which provides a broad-band, high gain antenna suitable for various types of television applications.
The disc-on-rod type antenna can have the discs spaced from each other over a wide range of values and can also be of a size covering a wide range. However, in my copending application Ser. No. 938,883, filed on Sept. 1, 1978, and also incorporated herein by reference, there is described a disc-on-rod end-fire antenna having a particular unique smaller range within the general larger range recited in the aforementioned issued patents, in order to provide a significant improvement. In particular, there is employed an end-fire radiator of a length between three times the wavelength and twelve times the wavelength, having a principal axis adapted to be energized by a launcher at its non-radiating end for the transmission of energy of a desired wavelength in the direction of the axis. The electrically active components of the radiator consist of the plurality of substantially identical thin electrically conductive plates spaced between 0.16 and 0.20 times the wavelength along the axis. The plane of the plates are normal to the axis and the difference between the diameter of the plates and the diameter of the supporting rod is greater than 0.23 times the wavelength and less than 0.27 times the wavelength.
In each of the aforementioned types of disc-on-rod antennas, a launcher is utilized to excite the antenna array. Usually, the launcher itself is an elementary antenna of the probe excited open-end waveguide type. More specifically, the launcher generally is formed of a metallic container having a closed back, side walls and an open mouth. A probe, usually formed of a coaxial transmission line, extends into the waveguide container through one of its walls. The probe serves to excite the launcher along the axis of the container. The disc-on-rod end fire radiator is connected along the principal axis of the launcher and is energized by the launcher to the particular energy wavelength. While such elementary antennas are useful as a launcher for the disc-on-rod type antenna, they can actually be utilized independently as a probe excited open-end waveguide for the transmission of energy by themselves.
In both the disc-on-rod type antenna, as well as the elementary antenna itself, additional electronic circuits are of course required to utilize the antenna for actual reception or transmission. For example, a preamplifier, a down converter, etc. would be needed in conjunction with the antenna in order to utilize it in practice. Such electronic circuits are generally contained within a package in a separate housing positioned in an external location relative to the antenna. Generally, the antenna is mounted on a pole and is subject to wind loading. The electronics package with its own housing is also mounted on the pole. The electronics is connected to the antenna by means of a coaxial cable. In order to eliminate losses, the coaxial connection between the electronics package and the antenna is kept as short as possible. Thus, with two separate housings, specifically the antenna itself, and the separate electronics package housing, there exists additional problems since now there are two housings which are subject to wind load, external environmental damage, excessive weight on the support, packaging and shipping problems. etc.
In connection with MDS type systems, there is special difficulty in connection with the installation of the antenna and the separate electronics housing. The transmitted signal is subject to losses due to interferences from the topology of the area as well as from buildings, and structures. It is therefore necessary to thoroughly survey each site prior to installation of the antenna. A suitable site must be selected which is free of path barriers such as mountains, or high ridges, as well as intervening building structures. Frequently, numerous locations must be tested before a single receiving antenna can be installed. Such installation may then be made utilizing various types of supports such as chimneys, masts, telephone poles, tripods, etc. Since the electronics package is provided separate from and in spaced relationship with the antenna, a separate mounting must be provided for the electronics package in addition to mounting of the antenna. This results in additional cost and difficulty in providing installation of both the antenna housing as well as the electronics package housing.
Furthermore, especially in MDS type systems, the height of the antenna is a critical factor in achieving proper reception. In many cases, the antenna is raised in height in order to increase the signal level received. However, with increased height above the building on which it is located, there is increased wind effecting the antenna and increased susceptibility to environmental damage. With the presence of a separate housing for the electronics package, the antenna system suffers from greater wind loss and greater susceptibility to external environmental damage.